1. Field of the Invention
The present invention relates to a desensitizer for a lithographic printing plate and to a process for preparing a lithographic printing plate using the same and, more particularly, it relates to a desensitizer suitable for a lithographic printing plate prepared from a peel-apart developable light-sensitive lithographic printing plate material wherein a support, a photo-hardenable light-sensitive layer and a peel-apart transparent cover film are present in this order as necessary components, and to a process for preparing a lithographic printing plate using the same.
2. Description of the Prior Art
Recently, various types of light-sensitive lithographic printing plate materials have been developed. In particular, materials using a photo-polymerizable composition (a so-called light-sensitive resin, photo-polymer or photoresist) as a light-sensitive layer are known as light-sensitive printing plate materials having the advantages that imagewise exposure and development processing can easily be conducted, that the time necessary for the development and subsequent treatment up to printing is short, that development can be conducted in a stable manner, that the printed images accurately reproduce the original image, that they can be stored for a long time as a light-sensitive material, and that the lithographic printing plate produced, has an excellent durability enabling many impressions to be printed.
Illustrative photopolymers which can be used in these printing plates include those which contain a photodimerization type light-sensitive resin as a major component, such as a polyester of p-phenylene diacrylate and 1,4-cyclohexanediol as described in Belgian Pat. No. 696,533, and a reaction product between a phenoxy resin and cinnamic acid or a carboxylic acid having one or more unsaturated groups as described in U.S. Pat. No. 3,387,976; those which contain a polymerizable compound having an ethylenically unsaturated bond and a binder (polymer) as major components, such as those which contain a photo-polymerizable material represented by the combination of an ethylenically unsaturated compound (e.g., diethylene glycol diacrylate, triethylene glycol dimethacrylate, pentaerythritol triacrylate, etc. as described in U.S. Pat. No. 3,043,805) and a binding agent (e.g., a methyl methacrylate/methacrylic acid copolymer, a styrene/itaconic acid copolymer, etc.) as major components.
Light-sensitive lithographic printing plates having these photo-polymers are imagewise exposed through an original image with a desired negative pattern, and developed using a suitable developer of an organic solvent, an alkaline aqueous solution or the like. Thus, the ultraviolet light irradiated areas remain on the plate support and the unexposed areas are dissolved and removed with the developer to form a pattern or dots of an original for printing on the plate support. Thus, lithographic printing plates are prepared.
In recent years, recording materials capable of being developed in a dry process have been proposed in place of such solution-developing type printing plate materials. Of these materials, those described in Japanese Pat. No. 9,663/63, U.S. Pat. No. 3,353,955, British Pat. No. 1,360,081 and Japanese patent application (OPI) No. 46,315/75 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) utilize the photo-polymerizable property of a photo-polymer and the change in adhesiveness due to the photo-polymerization. In these recording materials, a layer of a photo-polymerizable composition containing as major components a polymer (as a binder), an unsaturated monomer and a photo-polymerization initiator is provided on a synthetic resin film, metal, paper or like support, and a thin transparent film is superposed thereon as a cover. Printing plates are prepared from such a material by imagewise exposing the material through the cover, and then stripping or peeling off (delaminating) the cover therefrom to thereby leave either the exposed areas or unexposed areas of the light-sensitive layer on the support and the other areas on the cover, resulting in the formation of a negative image and a positive image (or a positive image and a negative image) on the support and the cover, respectively.
Examples of utilizing peel-apart developable light-sensitive materials using such a photo-polymerizable composition are described in Japanese patent application (OPI) Nos. 9,501/77 and 66,353/76.
Heretofore, a "desensitization (treatment)" has been generally used in the same sense as an "etching (treatment)" in the step of making an albumen plate or a deep-etch plate. The object of desensitization is to improve the hydrophilic property in the non-image areas, and a solution containing gum arabic and phosphoric acid as major components is often used as the desensitizer. After this desensitizing treatment, gumming with a solution containing gum arabic as a major component is usually conducted to protect the non-image areas.
On the other hand, with lighographic printing plates, non-image areas are usually densensitized, after development, with a solution of a hydrophilic, water-soluble and filmforming colloid like gum arabic. This desensitizing treatment renders the non-image areas highly hydrophilic, quite water-receptive, and oily ink-repellent in the presence of water.
This desensitizing treatment is based on the idea of (1) completely removing the light-sensitive layer which has not been removed on development, (2) rendering the metallic surface hydrophilic through a chemical treatment, and (3) enhancing the hydrophilicity of the metallic surface by adsorbing a hydrophilic colloid thereon. More specifically, a Cronak treatment, a Post-Nital treatment, a Brunak treatment, an etching solution treatment, a treatment with a hydrophilic colloid such as gum arabic, carboxymethyl cellulose, etc., or a combination of these treatments is often employed. This desensitizing treatment for deep-etch plates and plates which have been previously rendered light sensitive [presensitized (PS) plates] differs slightly. With deep-etch plates, counter-etching treatment with acetic acid or the like is conducted prior to coating a light-sensitive solution on the plate and, after plate-making, desensitization treatment is conducted by etching with a solution of a mixture of phosphoric acid and gum arabic. On the other hand, with PS plates, treatments for preventing a reaction between the light-sensitive layer and the aluminum surface are conducted. An alumite treatment, treatment with an aqueous zirconium fluoride solution, and treatment with an aqueous sodium silicate solution as described in U.S. Pat. No. 2,714,066 have been put into practice. In every case, the treatment is conducted before coating a light-sensitive solution on the plate. With PS plates too, gum-coating and desensitization with an aqueous solution of gum arabic alone are often conducted, after plate-making, as with deep-etch plates.
The objects of gumming are not only to protect the hydrophilicity of the non-image areas but to correct image areas such as retouching or elimination, to enable the plate to be stored after plate-making and before printing or to be stored for re-use, to prevent stains caused by adhesion of finger-prints, oils and fats, dirt, etc. during handling and mounting on a printing machine, to prevent formation of flaws or scratches, etc., and to control oxidation stains formed when the printing machines are stopped due to some difficulty or during rest periods.
Oxidation staining is the phenomenon of oxidation of the surface of the printing plate when the metallic surface of a printing plate comprising a metal plate, such as aluminum or zinc, is uncovered for a long time during the plate-making steps or printing steps, the surface is oxidized to form spot-like ink stains during printing, which can be prevented by gumming.
However, gumming using the conventional gum solution (desensitizer) described above is not completely satisfactory for peel-apart developable lithographic printing plates. The reasons for this are as follows. That is, when peel-apart developable lithographic printing plates are stored for a long time, innumerable "stain-like spots" are often formed in the non-image areas after imagewise exposure and development. The reason for this may be because a peel-apart developable type light-sensitive composition contains in many cases a hydrophobic liquid additive such as an ethylenically unsaturated monomer, and this liquid additive such as the monomer penetrates with time, when applied to a hydrophilic support for a lithographic plate, into the grains or fine pores on the surface of the hydrophilic support causing innumerable spotted stains on the surface of the hydrophilic support. Non-image areas must essentially be hydrophilic but, areas where such stains are formed are no longer hydrophilic and such stains cannot be removed or the areas cannot be rendered hydrophilic using a conventional gum solution which has the property of only forming a hydrophilic membrane on a printing plate. Therefore, when printing is conducted using a lithographic printing plate bearing such stains, serious printing stains will result (all over the surface).
In order to further increase the durability of a peel-apart developing type light-sensitive lithographic printing plate to meet the requirements for printing plates with high durability, the known process of irradiating the entire plate surface with actinic light after formation of printing images, a so-called post-exposure (or re-exposure), is effective.
In general, with solvent- or solution-developing type light-sensitive resin printing plates, it has been proposed, in order to obtain plates with an enhanced durability, to re-irradiate with actinic light (post-exposure) the entire light-sensitive resin printing plate, which has previously been subjected to image-forming exposure and to a treatment to dissolve away the unexposed areas, to further polymerize and complete hardening thereby increasing the hardness of the image areas. However, in post-treatment by post-exposure, the photo-hardening reaction is generally inhibited by oxygen in the air, and hence a mere irradiation of the plate surface with actinic light is not sufficient to completely harden the printing image areas and does not improve the durability of the printing plate.
Several methods have heretofore been proposed for removing these disadvantages. For example, a process of conducting the post-exposure in an inert gas such as carbon dioxide or nitrogen has been proposed. In this process, however, a gas-tight apparatus must be used, and a replacement of the atmosphere therein with the inert gas is necessary every time plates are changed. Therefore, this process is troublesome to conduct and is not an industrially practical process.
On imagewise exposing a peel-apart developable type PS plate, peel-apart developing the plate and irradiating the entire printing plate with actinic light (post-exposure), the following problems specific to peel-apart developable type PS plates are involved.
(1) When actinic light is applied in the presence of oxygen such as that in the air, polymerization is inhibited to such an extent due to the influence of oxygen that the effects of the actinic light can be scarcely obtained even when the irradiation is conducted for a long time.
(2) When irradiation with actinic light is conducted while eliminating the effects of oxygen, for example, in vacuo or in an atmosphere of carbon dioxide or nitrogen, polymerization proceeds in the image areas and, at the same time, polymerization of the light-sensitive composition or a thin layer comprising the light-sensitive composition which remains to some extent in the non-image areas takes place, and a strong oleophilic membrane is formed in the non-image areas. This results in extremely serious ink stains all over the plate surface when the plate is used for printing.
(3) The stains as described in (2) above are not generated when the irradiation with actinic light is conducted in an atmosphere as in (2) above after removing the remaining thin layer in the non-image areas with an organic solvent or the like. However, such a solution treatment after the peel-apart developing increases the number of development treatment steps and eliminates to a large extent the advantages of a peel-apart developing process as a dry process.
As a post-exposure process for removing the abovedescribed defects, the process of conducting the irradiation of the (lithographic) plate surface with actinic light, after peel-apart developing, in the presence of oxygen under heating, as described in Japanese patent application (OPI) No. 15,906/78, has been previously proposed.
However, this process has the defect that, since re-exposure is conducted without forming a hydrophilic membrane on a peel-apart developed lithographic plate, the plate cannot be subjected to a gumming treatment (for forming a hydrophilic membrane) immediately after the development as with ordinary PS plates or like plates. That is, when a peel-apart developable type PS plate is imagewise exposed, peel-apart developed and gummed with a gum commonly used for lithographic plates (e.g., a gum arabic aqueous solution) to provide a gum membrane on the lithographic plate surface, this gum membrane intercepts oxygen in the air upon post-exposure. Thus, the same problems as in (2) above occurs as to post-exposure. The image areas on the plate truely undergo sufficient photo-hardening but, at the same time, the components of the light-sensitive layer which remain to some extent in the non-image areas also undergo photo-polymerization, resulting in the non-image areas becoming oleophilic, which will cause undesirable printing stains.
Under such circumstances, development of a gum solution (desensitizer) which does not cause stains even when gumming is conducted immediately after peel-apart developing a peel-apart developable type PS plate and then post-exposure is conducted, and a process for post-exposing with sufficient post-exposure effects has been desired.